The invention relates to a relative-pressure sensor.
A distinction is made in pressure-measuring technology between differential-pressure, absolute-pressure and relative-pressure sensors. Differential-pressure sensors measure a difference between two different pressures. Absolute-pressure sensors measure pressures in absolute terms, that is, as a pressure difference with respect to a vacuum. A relative-pressure sensor measures a pressure difference with respect to a reference pressure. The reference pressure is an ambient pressure where the sensor is located. In most applications, this is the atmospheric pressure at the place of use. Relative-pressure sensors normally have a measuring chamber which is sealed with a pressure-sensitive measuring diaphragm. During operation, the pressure to be measured acts on the outside surface of the diaphragm. On a surface that opposes the diaphragm, the chamber has an opening by which the reference pressure is presented to the interior of the chamber at the measuring diaphragm. A transducer is provided which converts a deflection of the diaphragm into an electrically measured variable which is a function of the reference pressure and of the pressure to be measured.
A disadvantage of such sensors is that pollutants and/or moisture which impair the measuring accuracy may pass into the measuring chamber through the opening which supplies the reference pressure. Pollutants can be, for example, dust particles contained in the air, but also suspended or etching matter which is present at the place of use. If the temperature in the surroundings is higher than the temperature in the interior of the chamber, then condensate may form in the interior of the chamber.
Electromechanical transducers used to detect the deflection of the measuring diaphragm are generally very sensitive to pollutants and/or moisture.
EP-B 524 550 describes a relative-pressure sensor having a gas-filled measuring chamber which is sealed with a pressure-sensitive measuring diaphragm, on the outside of which a pressure to be measured acts during operation, and a transducer for converting into an electric measured variable a deflection of the diaphragm dependent on the pressure to be measured.
EP-B 524 550 also describes a second gas-filled chamber which is sealed with a second pressure-sensitive diaphragm on whose outside the reference pressure acts during operation. A second transducer is provided for converting a pressure-dependent deflection of the second diaphragm into an electric variable. The measuring chamber and the second chamber are interconnected via a line. The volume of the two chambers is, however, sufficiently large for the deflections of the measuring diaphragm and of the second diaphragm to be largely independent of one another. The relative-pressure sensor therefore effectively comprises two separate sensors of which one senses the pressure to be measured, and the other senses the reference pressure. The measured variables determined independently of one another are subsequently combined with one another to determine the relative pressure.
A disadvantage of such a relative-pressure sensor is that two transducers are required in order to detect the reference pressure and the pressure to be measured independently of one another. This means a higher outlay and higher costs in production.
A further disadvantage is that the two chambers have a large internal volume in order to decouple the two sensors. The gas quantity contained therein is correspondingly large. The pressure in the interior of the chambers rises or drops with temperature. A higher or lower pressure in the interior leads to a deflection of the two diaphragms and to measuring errors. It should be appreciated that the smaller at least one of the pressures to be determined, the stronger the effect of this source of error. The measuring errors of the two pressure measurements cumulatively effect this form of determination of relative pressure.
A differential-pressure sensor having two oil-filled chambers interconnected via a conduit is described in U.S. Pat. No. 4,425,799. Each chamber is sealed with a pressure-sensitive diaphragm. During operation, one of the pressures whose difference is to be measured works on each diaphragm. The oil serves as pressure transmitter, and has a substantially smaller coefficient of thermal expansion than air. There is thus a need for only one transducer, which detects the deflection of one of the two diaphragms which depends on the pressure difference, and converts the detected deflection into an electric variable.
However, there are many applications in which, for reasons of safety, the use of oil-filled sensors is precluded, as there is a risk that a diaphragm may be damaged and oil may escape. Examples of such applications can be found, in particular, in the food industry and in the production and storage of paints.
It is an object of the invention to specify a relative-pressure sensor which is protected against pollutants and/or moisture and manages without an oil filling.
For this purpose, the invention comprises a relative-pressure sensor having a gas-filled measuring chamber, which is sealed with a pressure-sensitive measuring diaphragm, on the outside of which a pressure to be measured acts during operation, a gas-filled compressible resilient bellows, on which a reference pressure acts from outside during operation, whose internal volume depends on the reference pressure, and which is connected to the measuring chamber in order to adapt an internal pressure prevailing in the measuring chamber to the reference pressure, and a transducer for converting a pressure-dependent deflection of the measuring diaphragm into an electric measured variable.
In accordance with another preferred embodiment of the invention, the bellows comprises two diaphragms, preferably corrugated diaphragms, interconnected at their outer edge.
In accordance with a further embodiment of the invention, the measuring diaphragm is arranged on a basic body, the basic body has a continuous bore, and the bellows includes a corrugated diaphragm which is connected with an outer edge to a rear side of the basic body averted from the measuring diaphragm and covers the bore.
In a still further embodiment of the invention, the bellows has a stiffness which is small by comparison with a stiffness of the measuring diaphragm.
In a particular and preferred embodiment of the invention, the bellows comprises a high-grade steel or a copper-beryllium alloy.